Reduced loss high efficiency diffractive and associated methods

ABSTRACT

A sub-wavelength anti-reflective diffractive structure is incorporated with a base diffractive structure having a small period to form a high efficiency diffractive structure. In the high efficiency diffractive structure, the anti-reflective structure and/or the base diffractive structure are altered from their ideal solo structure to provide both the desired performance and minimize reflections.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional application based on application Ser. No.11/391,486, filed Mar. 29, 2006, which in turn is a division ofapplication Ser. No. 10/231,485, filed Aug. 30, 2002, now U.S. Pat. No.7,064,899 B2, the entire contents of both of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a high efficiency diffractivehaving reduced reflection losses.

2. Description of Related Art

The use of a diffractive to split off a portion of an input beam formonitoring of power, wavelength or for other purposes is known. Oftenthe percentage of light to be split off is very small, e.g., a couple ofpercent of the input light. This is due to the fact that typically mostof the light is to proceed on to the actual application, and as muchpower as possible is to be preserved in the application beam. For suchlow percentage splitters, the period of the diffractive structureusually needs to be very small to eliminate excess loss to other orders.In other words, the structure needs to be small enough such that allorders above the ±1 orders are excluded in both the reflective andtransmissive mode. When a diffractive having such a very small period iscoated with an anti-reflective (AR) coating to reduce reflection losses,the performance of the diffractive is often degraded. This degradationis typically due to the fact that the AR coating coats the walls of thediffractive as well as the planar surfaces thereof. For a small periodstructure, the AR coating is thick enough relative to the period of thestructure that the AR coating degrades its performance.

SUMMARY OF THE INVENTION

The present invention is therefore directed to a high efficiencysplitter having reduced reflection losses and associated methods whichsubstantially overcomes one or more of the problems due to thelimitations and disadvantages of the related art.

It is an object of the present invention to integrate a sub-wavelengthanti-reflective diffractive structure with a base diffractive structurehaving a small period on the same surface to provide a high efficiencydiffractive structure.

At least one of the above and other objects may be realized by providingdiffractive structure including a base diffractive structure whichprovides a desired function at a design wavelength, the base diffractivestructure being formed on a surface, the base diffractive structurehaving a period on the order of the design wavelength, and ananti-reflective diffractive structure integrated with and on the samesurface as the base diffractive structure, the anti-reflective structurehaving a period that is smaller than the design wavelength.

The base diffractive structure may include a one-dimensional array ofsteps. The features of the anti-reflective diffractive structure mayform a one dimensional array parallel to the one dimensional array ofsteps. The features of the anti-reflective diffractive structure mayform a one dimensional array orthogonal to the one dimensional array ofsteps. The features of the anti-reflective diffractive structure mayform a two dimensional array. The base diffractive structure and theanti-reflective structure may be created simultaneously on the surface.The anti-reflective structure may be created on the surface before thebase diffractive structure is created on the surface. The basediffractive structure may be a splitter. The features of the basediffractive structure and features of the anti-reflective diffractivestructure may be orthogonal to one another in an elongated dimension.The anti-reflective diffractive structure may be etched into thesurface. The base diffractive structure and the anti-reflectivediffractive structure may be etched into the surface. At least one ofthe base diffractive structure and the anti-reflective structuredeviates from an optimal design for that structure alone.

At least one of the above and other objects may be realized by providinga diffractive structure including a base diffractive structure whichprovides a desired function at a design wavelength, the base diffractivestructure being formed on a surface, and an anti-reflective diffractivestructure integrated with and on the same surface as the basediffractive structure, the anti-reflective structure having a periodthat is smaller than the design wavelength, an etch depth of theanti-reflective structure being on an order of an etch depth of the basediffractive structure.

At least one of the above and other objects may be realized by providinga diffractive structure including a base diffractive structure whichprovides a desired function at a design wavelength, the base diffractivestructure being formed on a surface, and an anti-reflective diffractivestructure integrated with and on the same surface as the basediffractive structure, the anti-reflective structure having a periodthat is smaller than the design wavelength, wherein at least one of thebase diffractive structure and the anti-reflective structure deviatefrom an optimal design for that structure alone.

At least one of the above and other objects may be realized by providinga method for creating a high efficiency diffractive structure includingdesigning a base diffractive structure providing a desired function,designing an anti-reflective diffractive structure, combining designsfor the base diffractive structure and the anti-reflective diffractivestructure to form a combined design, optimizing the combined design foracceptable performance of the desired function and minimized reflection,the optimizing includes altering at least one of a depth of the basediffractive structure, a depth of the anti-reflective diffractivestructure, and a period of the anti-reflective diffractive structure, todetermine an optimized design, the optimized design deviating from thecombined design, creating at least one mask in accordance with theoptimized design, and patterning a resist from the at least one maskusing a lithographic technique in accordance with the optimized designfor each mask of the at least one mask to form the high efficiencydiffractive structure.

The creating may include creating at least two masks, one of the atleast two masks having information for only the base structure andanother of the at least two masks having information for only theanti-reflective structure. The at least one mask has information forboth the base structure and the anti-reflective structure.

These and other objects of the present invention will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating the preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will bedescribed with reference to the drawings, in which:

FIG. 1A is a side view of a generic design of a base diffractivestructure;

FIG. 1B is an elevational perspective view of the structure of FIG. 1A

FIG. 2A is an elevational perspective view of a first embodiment of asub-wavelength structure to be incorporated with the base diffractivestructure of FIGS. 1A-1B;

FIG. 2B is an elevational perspective view of a second embodiment of asub-wavelength structure to be incorporated with the base diffractivestructure of FIGS. 1A-1B;

FIG. 2C is an elevational perspective view of a third embodiment of asub-wavelength structure to be incorporated with the base diffractivestructure of FIGS. 1A-1B; and

FIG. 3 is an elevational perspective view of a sub-wavelength structureof the present invention incorporated with a base diffractive structure.

DETAILED DESCRIPTION

The present invention will be described in detail through embodimentswith reference to accompanying drawings. However, the present inventionis not limited to the following embodiments but may be implemented invarious types. The preferred embodiments are only provided to make thedisclosure of the invention complete and make one having an ordinaryskill in the art know the scope of the invention. The thicknesses ofvarious layers and regions are emphasized for clarity in accompanyingdrawings. Also, when a layer is defined to exist on another layer or asubstrate, the layer may exist directly on another layer or substrate,or an interlayer layer may be present therebetween. Throughout thedrawings, the same reference numerals denote the same elements.

FIG. 1A is a side view of a base diffractive structure splitter 10, herea splitter for creating two beams. The splitter 10 has a plurality ofsteps 12, 14 in a substrate 5. Each step 12, 14 has a height and awidth, which are determined in accordance with a desired performance andfunction at a design wavelength. Each step also has attendant side walls13, 15, 17, which can be seen more clearly in FIG. 1B, which is aperspective top view of the splitter 10. As can be seen from theseviews, if a coating were provided on this structure 10, the coatingwould also coat the side walls 13, 15, 17 of the structure 10, as wellas the planar surfaces of the structure. The features of thisdiffractive are so small that the linewidth of them is on the order ofthe thickness of a typical anti-reflective (AR) coating. In other words,the AR coating has a thickness which is a substantial percentage, e.g.,10% or greater, of the feature size. Thus, providing an AR coating onsuch structures degrades performance of the diffractive.

In accordance with the present invention, rather than using a coating,anti-reflective sub-wavelength diffractive structures are integratedwith a base diffractive structure. The base diffractive structureprovides the desired function and performance, while the anti-reflectivediffractive structures reduce reflections from the base diffractivestructure. These diffractive structures are on the same surface and forma single composite diffractive structure. These diffractive structuresmay be integrated by creating the anti-reflective diffractive structureon the surface at some stage during the base diffractive structurecreation or by incorporating the design of the anti-reflectivediffractive structures with that of the base diffractive structure andcreating them simultaneously. These structures may be createdlithographically in known manners. Either the anti-reflective structureor both diffractive structures may be etched into the substrate.

The base diffractive structure may have a period Λ_(sp) on the order ofthe design wavelength, e.g., λ<Λ_(sp)<10λ, often 2λ/n₁, where λ is thewavelength of interest and n₁ is the refractive index for the mediuminto which the light is transmitted. This period is small enough toeliminate excess loss in the higher orders, i.e., above the ±1 orders.The period is determined by the desired angles to be output from thesplitter. The depth of the base diffractive structure is small, i.e.,consistent with most of the light being transmitted into the zero^(th)order. The AR structure may have a period Λ_(AR) that is less thanλ/max(n₀,n₁), where n₀ is the refractive index of the medium in whichthe light is traveling. An ideal AR structure would provide an effectiverefractive index at the interface of approximately √n₀n₁. The depth ofthe AR structure is of the same order as that of the base diffractivestructure. Since the base diffractive structure and the AR structure areof similar feature size, a mere additive combination of these structureswould not result in an optimized high efficiency diffractive structure.Therefore, the ideal base diffractive structure and the ideal ARstructure are first combined. The resultant structure is then modeled ina known fashion. Then at least one of the depth of the base diffractivestructure, the depth of the AR structure, and the period of the ARstructure, or any combination thereof, is altered until an optimizedhigh efficiency diffractive structure is realized, i.e., a structurethat provides both acceptable desired performance and minimizesreflections. In this manner, the base diffractive portion may be alteredin order to improve performance due to the presence of the AR portion.

A mask or set of masks is then created in accordance with this optimizeddesign and used to lithographically create the high efficiencydiffractive structure. The mask(s) set the period for the structure. Thedepths of either structure may be altered to meet the optimized designby changing the exposure times and/or parameters of the resist beingexposed. Depending on the particular optimized structure, one or more ofthe masks may have information regarding both the diffractive structureand the AR structure, or each mask may only contain information aboutone of the diffractive structure and the AR structure. The samelithographic equipment may be used for transferring the pattern for eachmask into resist on a substrate. The pattern in the resist may then betransferred into the substrate. A plurality of high efficiencystructures may be created on the wafer level and then singulated.Examples of different sub-wavelength AR structures are discussed below.

A first embodiment of a diffractive sub-wavelength AR structure 20 isshown in FIG. 2A. The AR structure 20 includes sub-wavelength steps 22that run in the same direction as the steps 12, 14 of the base splitterstructure 10. Due to the fact that these features are in the samedirection, they may interfere with the desired functioning of thesplitter 10, due to possible layer-to-layer misalignment. The effect ofany such misalignment may be minimized by selecting the period of the ARstructure such that the period of the base diffractive structure is notan integer multiple of the period of the AR structure.

A second embodiment of a diffractive sub-wavelength AR structure 30 isshown in FIG. 2B. Here, the AR structure 30 includes sub-wavelengthsteps 32 that run transverse to the steps 12, 14 of the base splitterstructure 10. Because the steps 32 are transverse, alignment should notbe as much of a concern. However, the attendant depth modulationintroduced by creating the AR diffractive structure 30 before thesplitter 10 may complicate the manufacturing process. The design of theAR diffractive structure 30 may be incorporated with the design of thesplitter 10 so that they are created simultaneously.

A third embodiment of an AR diffractive sub-wavelength structure 40 isshown in FIG. 2C. Here, the AR diffractive structure 40 includessub-wavelength steps 42 that do not extend continuously as the previoussteps, but form a discrete two-dimensional array. Since the featuresizes of this embodiment are smaller than the other embodiments, thesefeatures may be harder to create. Alignment issues may also present aproblem. However, these steps are less polarization sensitive thanlines, which may be birefringent.

FIG. 3 illustrates the splitter 10 of FIGS. 1A-1B with a transverse ARdiffractive sub-wavelength structure 30 of FIG. 2B incorporated thereinto form a reduced loss, high efficiency splitter 50. As can be seen bycomparing the structure 50 in FIG. 3 with the structure 10 in FIGS.1A-1B, the incorporation of the AR sub-wavelength structure 30 reducesthe height of the splitter structure at certain intervals.

In accordance with the present invention, by providing sub-wavelengthdiffractive structures to serve as anti-reflection features, reflectionlosses may be reduced in a small period diffractive structure withoutincurring the attendant problems with coating the small perioddiffractive structure.

Although preferred embodiments of the present invention have beendescribed in detail herein above, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptstaught herein, which may appear to those skilled in the art, will stillfall within the spirit and scope of the present invention as defined inthe appended claims and their equivalents.

1-17. (canceled)
 18. A diffractive structure, comprising: a surface onwhich an integrated diffractive structure is formed in accordance withan integrated diffractive design, the integrated diffractive designbeing formed from: a base diffractive design which provides a desiredfunction at a design wavelength; an anti-reflective diffractive designhaving a period that is smaller than the design wavelength; and theintegrated diffractive design formed by deviating at least one of thebase diffractive design and the anti-reflective design from a design forthat structure alone.
 19. The diffractive structure as claimed in claim18, wherein the base diffractive design has a period on the order of thedesign wavelength.
 20. The diffractive structure as claimed in claim 18,wherein a depth of the anti-reflective design is on an order of a depthof the base diffractive design.
 21. The diffractive structure of asclaimed in claim 18, wherein the base diffractive design includes aone-dimensional array of steps.
 22. The diffractive structure as claimedin claim 21, wherein features of the anti-reflective diffractive designform a one dimensional array parallel to the one dimensional array ofsteps.
 23. The diffractive structure as claimed in claim 21, whereinfeatures of the anti-reflective diffractive design form a onedimensional array orthogonal to the one dimensional array of steps. 24.The diffractive structure as claimed in claim 18, wherein features ofthe anti-reflective diffractive structure form a two dimensional array.25. The diffractive structure as claimed in claim 18, wherein deviatingincludes varying at least one of a depth of the base diffractive design,a depth of the anti-reflective diffractive design, and a period of theanti-reflective diffractive design.
 26. The diffractive structure asclaimed in claim 18, wherein the base diffractive design is a splitter.27. The diffractive structure as claimed in claim 18, wherein featuresof the base diffractive design and features of the anti-reflectivediffractive design are orthogonal to one another in an elongateddimension.
 28. The diffractive structure as claimed in claim 18, whereinthe integrated diffractive structure is on the surface.
 29. Thediffractive structure as claimed in claim 18, wherein the integrateddiffractive structure is in the surface.
 30. A diffractive structure,comprising: a surface on which an integrated diffractive structure isformed in accordance with an integrated diffractive design, theintegrated diffractive design being formed from: a base diffractivedesign which provides a desired function at a design wavelength; ananti-reflective diffractive design having a depth on an order of a depthof the base diffractive design; and the integrated diffractive designformed by deviating at least one of the base diffractive design and theanti-reflective design from a design for that structure alone.
 31. Thediffractive structure as claimed in claim 30, wherein deviating includesvarying at least one of a depth of the base diffractive design, a depthof the anti-reflective diffractive design, and a period of theanti-reflective diffractive design.
 32. The diffractive structure asclaimed in claim 30, wherein the base diffractive design is a splitter.33. The diffractive structure as claimed in claim 30 wherein features ofthe base diffractive design and features of the anti-reflectivediffractive design are orthogonal to one another in an elongateddimension.
 34. The diffractive structure as claimed in claim 30, whereinthe integrated diffractive structure is on the surface.
 35. Thediffractive structure as claimed in claim 30, wherein the integrateddiffractive structure is in the surface.